Yes, it is very efficient. It does 150mh/s slightly overclocked(850 core) and it is only 50$! So you can get 600mh/s for a mere 200$. 6870 does that for 250$! But before you run to your local store, be sure to buy Powercolor ones because they're slightly different from regular 5670's. Here be pics:
http://i.imgur.com/kWaNkdY.png
running at 290mh/s

Probably the "Juniper" text caught your eye. That's why I said that the Powercolor ones are different! Now go and buy some 5670's!